Endless track-driven vehicles are commonly used off-road in difficult terrain and under difficult terrain conditions, such as in mud, snow, sand, and tundra. For example, tracked vehicles are used in snow country for grooming ski slopes and snow mobile trails, for transporting skiers to back-country slopes, for ski resort maintenance work, and for snow and mountain rescue. They are also used in various types of terrain for utility company maintenance work, and for oil exploration and oil pipeline maintenance in arctic tundra.
Tracked vehicles are generally of two types. Many are two-tracked in which a pair of endless drive track units, one on each of the opposite sides of the vehicle, support and drive the vehicle. Others are four-tracked, in which four separately driven and independently suspended drive track units, two in front and two in the rear, support and drive the vehicle.
Four-tracked vehicles have certain advantages over two-tracked vehicles under extreme conditions such as on steep slopes and in very rough terrain because of the flexible independent suspensions of the track drive units and the constant power available to all of the track drive units, even while turning. Unlike a two-tracked vehicle which relies on the differential speed of the two tracks for turning, a four-tracked vehicle steers much like a wheeled vehicle. Its endless drive track units can be physically turned for steering.
In many tracked vehicles, such as the four tracked vehicles shown in U.S. Pat. No. 6,007,166, the lower, ground-engaging run of the track has been supported by a plurality of single-element guide wheels disposed substantially inline longitudinally of the track and generally engaging only a centralized region of the track. Although this system, with a single line or row of guide wheels, functioned adequately, it was found that substantial deflection of the track on opposite sides of the guide wheels was occurring. This deflection was caused generally by the high-point loading of the track by the guide wheels at the center of the lateral dimensions of the track. This could lead to premature failure of elements in the track due to high cyclical stresses. This same high-point loading of the track and its consequential deflection could also lead to premature failure of the track and reduce its effective traction. The high-point loading of the track can also be transmitted through the track to the underlying terrain. In the case of sensitive terrain such as tundra, such loading could cause excessive damage to the terrain, especially with an endless track that includes traction bars or cleats on its outer surface for enhancing the vehicle's traction.
The drive track unit disclosed in U.S. Pat. No. 6,129,426, addressed the foregoing problems by providing guide wheel assemblies along the lower track run, with each assembly including multiple guide wheels mounted on a common guide wheel hub. This increased the guide wheel surface contact area across the width of the track, thereby reducing point loading of the track and consequential track deflection and wear, as well as terrain damage. Although this was a substantial improvement over the prior art, a problem with such guide wheel assemblies is that snow and/or ice can adhere to the outer surface of the hub between the guide wheels. Once a snow/ice layer is formed over the hub, additional snow and/or ice can quickly accumulate between the wheels, which can cause the track to “derail” or disengage from the guide wheels.